In modern surgery, one of the most important instruments available to medical personnel is the powered surgical tool. Often this tool is in the form of a drill unit in which a motor is housed. Secured to the drill unit is a cutting attachment designed for application to a surgical site on to perform a specific medical procedure. For example, some powered surgical tools are provided with drills, burs or reamers for cutting bores into tissue or for selectively removing tissue such as bone. Other powered surgical tools are provided with saw heads. These tools separate large sections of hard and soft tissue. A wire driver is a power tool that, as its name implies, drives a wire into a patient, more particularly, a bone. Power tools are also used to perform other functions in the operating room. For example, it is known to use a power tool to mix the components that form a mass of surgical cement.
The ability to use powered surgical tools on a patient lessens the physical strain of surgeons when performing medical procedures on a patient. Moreover, most surgical procedures can be performed more quickly and more accurately with powered surgical tools than with the manual equivalents that preceded them.
One type of powered surgical tool that is especially popular with some physicians is the cordless, battery-operated powered surgical tool. As the name implies, this tool has a battery that serves as the power source for the motor. This eliminates the need to provide the tool with a power cord connected to an external-power source. Elimination of the power cord offers benefits over corded, powered surgical tools. Surgical personnel using this type of tool do not have to concern themselves with either sterilizing a cord so the cord can be introduced into the sterile surgical field or ensuring that, during a procedure, an unsterilized section cord is not inadvertently introduced into the surgical field. Elimination of the cord also results in the like elimination of the physical clutter and field-of-view blockage a cord brings to a surgical procedure.
One feature shared by both corded and cordless power surgical tools is the presence of a control switch or member on the tool. This member is often in the form of a biased switch, trigger or button. A number of corded and cordless surgical tools have handles similar to pistol handgrips. A tool of this shape is sometimes designed so the control member is trigger that is slidably mounted to the handle.
Powered surgical tools, unlike many other power tools, have to do more than deliver relatively large amounts of power. A powered surgical tool must be able to withstand repeated exposure to an environment both saturated with water vapor and very hot. This is because, prior to use, a powered surgical tool is autoclave sterilized. In this process, the tool is placed in a chamber where there is atmosphere is saturated with water vapor (steam), the temperature is approximately 270° F. and the atmospheric pressure is approximately 30 psi (Gage). Internal components of the tool, including the conductive components of its control circuit, if left unprotected in and repeatedly exposed to this environment, corrode.
The Applicant's U.S. Pat. No. 5,747,953, CORDLESS, BATTERY OPERATED SURGICAL TOOL, issued May 5, 1998, and incorporated herein by reference, discloses one means for protecting the internal components of a surgical tool from the affects of autoclave sterilization. The tool of this invention has a sealed module that houses the circuit that regulates tool actuation. Also internal to this module are contactless sensors that monitor the states of externally mounted triggers. Attached to each trigger and located inside the tool housing is a magnet. Internal to the module are magnetic field sensors. Each sensor generates a varying signal as a function of the proximity of an associated one of the trigger magnets. The manual displacement of the trigger results in a like displacement, inside the tool, of the magnet. When a trigger and magnet are so displaced, the complementary sensor generates a signal that indicates the movement has occurred. Upon receipt of this signal, the control circuit generates the signal needed to allow an energization current to be applied to the motor.
The electrically conductive components of the on/off control assembly of the above tool are shielded from the supersaturated heated air of the autoclave environment. When this tool is sterilized, these components are not adversely affected.
However, known cordless power tools have other sensitive components that remain exposed. These components typically include the sensors that monitor the operation of the power-producing units. Many motorized cordless power surgical tools, for example, employ brushless DC-motors as their power-producing units. Internal to this type of motor are sensors that monitor the position of the motor's rotor. The signals produced by the sensors are supplied to the control circuit. These signals function as feedback signals that, with the on/off signals, regulate the commutation of the motor.
These sensors are exposed to the corrosion fostering environment of the autoclave. Currently, these sensors are encased in a potting compound to shield them from the harsh effects of the sterilization process. Nevertheless, over time, pressurized water vapor can reach these sensors. Once this occurs, the water vapor has a tendency to corrode the sensors so as to cause their malfunction.
Even when these sensors remain shielded from the saturated water vapor, there are some disadvantages associated with their use. Often, these sensors operate best in low temperature environments. For example, the signals generated by Hall effect sensors start to vary at temperatures above 150° C. The motor integral with a powered surgical tool can generate enough heat to cause the temperature to rise above this level. Once this occurs, the variations in the signals output by the Hall sensors can cause the control circuit to generate control signals that foster tool malfunction.
Moreover, the accuracy of the motor rotor position signals generated by these sensors is naturally very dependent on sensor position relative to the rotor. Despite the best efforts of surgical personnel, it is not unheard of for surgical tools to drop to the floor. When a tool is exposed to this type of mechanical shock, the positions of the motor rotor sensors can shift. Such movement is still another reason why the sensors may to generate signals that do not accurately represent motor rotor position.
In theory, it should be possible to eliminate this problem by using the back EMF signals generated by the motor windings to obtain an indication of rotor position. This is how use of rotor position sensors in corded powered surgical tools is eliminated. In practice, it has proven difficult to implement this solution in a cordless powered tool. This is because, at zero speed, stall speed, there are no back EMF signals from which rotor position can be determined. Instead, other means are employed to energize the motor windings in order to start up the motor. These other means typically involve the application of significant currents to the windings. During a surgical procedure, a cordless power tool may be repeatedly cycled on and off. Therefore, if a cordless powered surgical tool were driven based on the state of back EMF signals, the power required to constantly restart the motor can result in relatively rapid depletion of the battery charge. This could require the battery to be changed in the middle of the procedure. Clearly, this is a task surgical personnel would like to avoid.
Moreover, many powered surgical tools, both of the corded and cordless variety, drive different cutting accessories. For example, many drill units are designed to drive both shavers and burs. Often, different accessories operate at different preferred speeds have different maximum operating speeds. A number of different assemblies are commercially available that provide feedback to the control console that energizes a corded power tool to indicate the type of attached cutting accessory. Based on this information, the control console regulates actuation of the tool so it operates at speeds appropriate to the attached accessory. However, a cordless power tool does not have a control console. Thus, it has proven difficult to provide a mechanism that can be used to custom regulate the operation of the tool based on the attached accessory.
Moreover, some corded powered surgical tools have control consoles able to provide custom speed or operation settings based on surgeon preference. Again, since a cordless tool is not connected to this type of console, it has proven difficult to provide surgeons with this type of control with this type of tool.